1. Technical Field
The present disclosure relates to wideband transmitters, and more particularly to mixers and baluns used in wideband transmitters.
2. Background Information
Many types of RF (Radio Frequency) transceivers, including cellular telephone transceivers such as in multi-band cellular telephones, must work over a wide frequency range. In one example, a cellular telephone may be required to operate in a first frequency band referred to here as a “cell band” or a “low band”. The same cellular telephone may also be required to operate in a second frequency band referred to here as a “PCS band” or a “mid band”. FIG. 1 (Prior Art) is a diagram that shows frequency along the horizontal axis. Low band 1 in this example extends from 824 MHz to 915 MHz and mid band 2 extends from 1710 MHz to 1980 MHz.
FIG. 2 (Prior Art) is a circuit diagram that illustrates a first way that a transmitter of a multi-band cellular telephone transceiver can be made to operate over the wide frequency range of FIG. 1. The transmitter includes a transmit baseband filter 3, a mixer 4, a balun 5, a driver amplifier 6, a power amplifier 7, a duplexer 8, and an antenna 9. The dashed box 10 indicates the portion of the transmitter that is realized on an RF transceiver integrated circuit. Balun 5 includes one primary winding 11 and one secondary winding 12. A first programmable capacitor 13 is coupled in parallel with the primary winding and a second programmable capacitor 14 is coupled in parallel with the secondary winding. To make the transmitter operable over the wide frequency range, the capacitors 13 and 14 are made to be large and tunable capacitors. Such a large and tunable capacitor typically involves banks of capacitors and associated transistor switches. The transistors are used to switch the capacitors in and out of the overall structure to increase or decrease the overall capacitance. Unfortunately, making the first and second capacitors large and programmable in this way reduces the quality factor (the “Q”) of the balun. Due in part to this low quality factor, the transceiver when transmitting in the low band may emit an undesirable amount of receive band noise in a nearby receive band even though the transmitter is tuned to transmit in a transmit band. The transmit and receive bands are typically quite narrow and are located quite close to one another within the wider low band or the wider mid band.
FIG. 3 (Prior Art) illustrates a transmit band 15 and a receive band 16 that may, for example, exist side by side in the low band 1 of FIG. 1. When the circuit of FIG. 2 is used to transmit in transmit band 15, an unwanted amount of energy is also transmitted into receive band 16 due to the low Q of balun 5 of FIG. 2.
FIG. 4 (Prior Art) is a diagram of a second way that a transmitter of a transceiver integrated circuit 32 of a multi-band cellular telephone can be made to operate over a wide frequency range such as the wide frequency range illustrated in FIG. 1. Because the balun tuning range is a function of both inductance and capacitance, if the tuning range of the balun capacitances is limited as in FIG. 2 due to quality factor issues then an amount of inductance tuning is provided by providing two higher-Q baluns having different winding inductances. Accordingly, one transmit baseband filter 17 is provided, but the remainder of the transmitter is duplicated. A low band circuit path 18 involves mixer 19, balun 20, driver amplifier 21, power amplifier 22, and a duplexer 23. This low band circuit path 18 is optimized for operation in the low band of FIG. 1. A mid band circuit path 24 involves mixer 25, balun 26, driver amplifier 27, power amplifier 28, and a duplexer 29. This mid band circuit path is optimized for operation in the mid band of FIG. 1. The two mixers 19 and 25 are both driven by the same transmit local oscillator signal TX LO. An antenna switch 30 couples the appropriate one of the two circuit paths to antenna 31. If the transmitter is to transmit in the low band, then signal EN DA1 is asserted to enable driver amplifier 21 and signal EN DA2 is not asserted such that driver amplifier 27 is disabled. Conversely, if the transmitter is to transmit in the mid band, then signal EN DA2 is asserted to enable driver amplifier 27 and signal EN DA1 is not asserted such that driver amplifier 21 is disabled.
The two-path transmitter circuit of FIG. 4 does not have the low Q balun problem of the transmitter circuit of FIG. 2, but the two-path transmitter circuit of FIG. 4 is undesirably large as implemented due to the redundant circuitry. The two-path transmitter also consumes an undesirably large amount of power. Interconnections between the divider circuitry that generates the transmit local oscillator signal TX LO and the mixers can be long when there are two mixers 19 and 25. Such long interconnections often result in increased current consumption.